In roofing structures for industrial buildings, the outer skin is usually formed by roofing sheets, e.g. asbestos sheets, which are clamped by means of hook members to the turned back edges of the upper lateral flanges of the purlins. Conventionally, the purlins have a Z-shaped cross-section and are known as Z-purlins. At the present time, there invariably exists a requirement to incorporate insulation in the roof structure.